In a burner of this type the solid fuel is typically combusted on a grate where it is deposited by a fuel feeding mechanism, such as for example a screw conveyor including a screw disposed in a metal tube at one end connected to a silo or similar storage of solid fuel, the other end being positioned within the burner above the grate.
The grate is typically a stepped grate including a number of staggered grates which may be movable in relation to each other in order to displace ashes and other residual combustion products towards the bottom of the burner, from where they can be removed by an ash removal mechanism similar to the fuel feeding mechanism.
Air is supplied to the burner as primary air, i.e. air supplied to a space below the grate, wherefrom it passes upwardly through the grate and provides oxygen to the combustion process, and as secondary air, i.e. air which is provided to the space above the grate to provide further oxygen to the combustion of the fuel and any further combustion taking place in the flue gas produced by the combustion process. The primary and secondary air is typically supplied by electrical blowers.
In many cases the burner is combined with a boiler, whereby the flue gas resulting from the combustion process is led through flue gas pipes deposited in a boiler, where the heat of the flue gas is used to heat water circulating through the boiler. The cooled flue gas may then be released into the atmosphere via a chimney, possibly after further cleaning of the flue gas.
Typically, secondary air is supplied from two opposite directions, whereby the collision of the air streams forces part of the secondary air down through the grate, which hampers the flow of the primary air, thus affecting the oxygen dependent combustion negatively.
A burner where the secondary air is not supplied from two opposite directions is known from amongst others WO8301671; however, this burner supplies the secondary air as a horizontal rotational flow. Thus, the secondary air may still interfere with the primary air.
Other examples of burners can be seen in for example U.S. Pat. No. 4,697,530, which discloses an automatic stoker boiler having air openings in an air plenum, which are placed in the opposite side of the stoker as the fuel inlet. Especially the air openings are not placed around the fuel outlet, and WO 02/079693 discloses a burner with a fuel outlet and a grate positioned below said outlet. The burner is equipped with a number of air holes in all the sides for the supply of secondary air, for cooling the sides and cover part. By the disclosed arrangement of air holes in all sides of the burner it is not possible to sustain unidirectional parallel flow of air over the grate.
US 2007/089733 discloses a wood-burning boiler having a fire chamber and a loading door for fuel supply. The boiler also comprises a grate, under which under fire air is supplied for sustaining the combustion of the fuel. In the rear plate there is provided a plurality of air holes for the supply of secondary over-fire air into the fire chamber. The disclosed arrangement of air holes is therefore not capable of sustaining a unidirectional parallel flow of air over the grate.
GB 2 193 798 discloses a cylindrical coal burner equipped with a stepped grate, wherein each grate section is equipped with a plurality of venture ports, which are disposed in the side of the cylindrical wall of the burner, and at an angle of approximately 60 degrees relative to the longitudinal portion of the burner. In the end wall there is provided a plurality of air ports for the inlet of air in a direction substantially along the longitudinal extension of the burner. These air ports are not placed above the fuel outlet, and the combined effect of these air ports is to impart a circulatory motion to the coals.
FR 2 570 472 discloses a cylindrical coal stoker having a fuel outlet and an exhaust, a grate positioned below said outlet and between said fuel outlet and said exhaust. FR 2 570 472 also discloses means for supplying primary air through said grate for the combustion of coal, and means for supplying secondary air from a plenum above said fuel outlet.
WO 00/75563 discloses a burner for the burning of solid fuels. The burner comprises a fuel outlet and an exhaust, a cartridge with longitudinal apertures for the provision of under-fire air to enhance the combustion of the fuel. Furthermore, adjacent to the fuel outlet there are placed two tube pieces for the provision of air along the longitudinal direction of the burner. Additionally, there are provided air holes in the bottom of the burner for the provision of tertiary air into the combustion chamber in order to create turbulence within the combustion chamber. Moreover the tube pieces are placed so far apart from each other that they will not be able to sustain a unidirectional parallel flow of secondary air over the grate.